Process of reducing iron oxide



Oct. 17, 1967 w, VOLK ET AL 3,347,659

PROCESS OF REDUCING IRON' OXIDE Filed May 19, 1965 GAS RECOVERY RECYCLE INVENTOR CLARENCE A. JOHNSON WILLIAM VOLK United States Patent 3,347,659 PROCESS OF REDUCING IRON OXIDE William Volk and Clarence A. Johnson, Princeton, N.J., assignors to Hydrocarbon Research, Inc., New York, N.Y., a corporation of New Jersey Filed May 19, 1965, Ser. No. 457,048 2 Claims. (Cl. 75.5)

ABSTRACT OF THE DISCLOSURE Production of critical size and volume-weight molding powders is accomplished by low temperature high pressure fluidized reduction techniques followed by size grinding of pyrophoric reduced iron.

This invention relates to improvements in the production of metal powder for powder metallurgy and is particularly addressed to -a combined reduction and grinding step by which it is possible to obtain high yields of the desired density and size of powder, more especially iron.

It has been established that the grain size of powdered iron is of decisive importance in the production of articles by the practice of powder metallurgy. It has also been established that the powder must be, largely, of a size to pass through a 325 mesh (Tyler) screen and that it should have a volume-weight ratio to facilitate proper compacting to final shapes.

'It has also been established that iron ore or iron oxide can be effectively reduced in the presence of a reducing gas under fluidized conditions at temperatures below melting to produce a highly pure metal. For this reduction, we have found that it is essential that the ore or oxide be of such a size distribution that it can be readily fluidized by the passage of the reducing gas through beds of the ground ore or oxide. However, the requirements for size for fluidization are not compatible with the requirement for size for molding powder with the result that yields of satisfactory size product have been low.

It has been considered that for the production of iron powder of the desired grain size for powder metallurgical purposes, it was first necessary to mechanically reduce an iron oxide (or iron ore) to a particle size not greater than the desired grain size, after which the powder would be chemically reduced. Otherwise, grinding, after chemical reduction caused an objectionable change in the volumeweight ratio of the product.

We have found also that the grinding of powder heated above 1600 F. is not practical where the purpose is to change the ultimate particle size inasmuch as the iron tends to become malleable and smears on the grinders rather than being disintegrated. Such temperatures are usually required either for direct reduction or for passivation of powder which, due to lower temperatures of reduction, is

pyrophoric.

Our invention is based on the discovery of the unexpected results in grinding of pyrophoric iron powder produced by the low temperature, high pressure fluidized system, before passivation, under controlled conditions, whereby an acceptable product, all of which will pass the rigorous size requirements and have the proper volumeweight ratio is produced. This is apparently due to the greater brittleness of the pyrophoric powder compared to powder heated to elevated temperatures. The subsequent sintering and regrinding to size does not change the ultimate particle size.

3,347,659 Patented Oct. 17, 1967 The principal object of our invention is to produce high yields of ground pyrophoric iron powder, which is subsequently sintered and again ground to size for use in powder metallurgy.

Further objects and advantages of our invention will appear from the following description of a preferred form of embodiment thereof, taken in connection with the drawing attached and illustrative thereof, and in which the figure represents a schematic apparatus for carrying out the process of reducing iron ore or oxide to produce metallurgical powder.

The reduction of iron ore or oxide by a fluidized process is more clearly defined and described in the patent to Keith et al., 2,900,246; and Keith, 3,140,940. In such a process, the iron ore or oxide, suitably dried and ground as hereinbefore mentioned to all pass a 20 mesh (Tyler) screen with not more than about 25% passing a 325 mesh screen and with about 25 and not to exceed 40% larger than mesh is conveyed, as by dense phase gas transport through line 10 to the upper portion of a multi-bed reactor -12 as shown. The iron ore thus forms a bed 15 on upper partition 16.

In turn, by means of downcomers 20, 22 and 24, each having valves 28, the ore or oxide is sequentially moved downward to form beds on other partitions 16 through which a reducing gas, entering through valve 30 passes. The reducing gas having the highest purity thus acts on the bed of the lowest oxidation condition so that a highly reduced'iron is removed in line 26 through valve 29.

If desired, and in the interest of heat economy, and as disclosed in the Stotler patent, 2,805,144, the temperature in the separate ore beds may be controlled by internal heat exchange, suitable heat exchange fluids such as preheated hydrogen being circulated'through inlet headers 38, exchanger tubes 40 and outlet headers 42. Such exchangers may appear in each of the beds. Alternatively, dummy tubes 32 carried by header 27 may be used as in the lower bed, all in the interest of establishing a desired baflie arrangement as described in Keith patent, 2,995,426.

Temperature and pressure conditions in reactor 12 are preferably as set out in the Keith et a1. patent, 2,900,246.

Pressures are preferably in the range of about 200 to 650 pounds per square inch gauge (p.s.i.g.), and temperatures for a typical hematite, magnetite or mill scale are usually in the range of 600 F. to 1100 F., preferably about 850 F. to 950 F. The hydrogen should have a purity of at least 60% with a water content not to exceed about 3.0% by volume. The upflow gas velocity is in the range of 0.5 to 5.0 feet per second (superficial).

Following reduction in the separate beds, the reduced iron which is pyrophoric is conveyed as by dense phase transport through line 26 to closed storage hopper 44. This hopper is provided with a discharge pipe 48 having control valve 46 by which the feed of reduced iron particles may be controlled in the flow to the grinding operation 50.

The random size iron particles are now ground to a predetermined size in grinding mill 50 which may be of the ball type. This is preferably shielded with a nitrogen gas. 'The ground product is next passed to screen 52 from which the fines are removed at 54 and the coarser particles are removed at 56. The coarse particles can then be recycled to the grinding mill 50 and reground so that only fines will be produced. Usually, all product will pass 100 mesh and usually about 60-80% will pass 325 mesh.

3 r 4 The following table gives .the results of four tests on about 25 percent and about 40 percent being larger screened and ball milled reduced mill scale: than 100 mesh;

(b) reducing said iron oxide in a fluidized bed at a tem- Run NO perature in the range of 600 F. to 1100 E, and at a 5' pressure in the order of 200 to 650 p.s.1.g., with a I 11 111 IV hydrogen rich reducing gas by passing the reducing gas upwardly through the iron oxide at a velocity to geductlim e r- 5 ei gg gistablishflllidilationil d d d f Greene c removing a pyrop one re uce iron pow er rom BllNllldh. s 5 5 sr ve ri n l is rmduct T ler 10 said fluidized bed;

Mesh); 1 0 1 (d) grinding said pyrophoric reduced iron powder with- 012 011 out prior passivation to all pass 100 mesh and with 8% 8% 60-80% passing 325 mesh; 1 (e) sintering said ground powder from step (d) at a g; 2-2 temperature in the range of 1600 to 2 200 F.;

1 (f) and regrinding said sintered material from step (e) 2 5 3 3 2 31 2 23 to size. dig; iiengiiii gfsi fl fifiI 2, '250 2,'19o 2,515 2, 360 2. The method of mak ng g Yields of highqllality i f i g [c 6 36 6 38 6 43 6 40 fines as claimed in claim 1 wherein a coarse material havrfinihe stresgfiaj5.53:3: 21,500 21,500 21,300 22,800 20 ins err e e greater t 100 mesh is screened Elmlgfitinn, Percent 7 8 from the remaining reduced iron and is recycled to the grinding step. Sintering is accomplished at a temperature in excess of References Cited 1600 F. and usually not above 2200" F. but this does UNITED STATES PATENTS not objectionably affect the ground powder for powder 25 2721135 10/1955 Wimberly 75 55 metallurgy Purposes- 2,857,270 10/1958 Brundin 75 .5 While we have shown and described a preferred form 2 860 044 11/1958 Brundin 5 of embodiment of our invention, we are aware that modi- 2954288 9/1960 Amen fications may be made thereto within the scope and spirit of the description herein and of the claims appended here- 319'9974 8/1965 Johnson et a1 7526 inafter. OTHER REFERENCES We claim: I Goetzell, Claus G.: Treatise on Powder Metallurgy,

1PTh method of making high Yields of h quality 1949, vol. I (Technology of Metal Powders and Their fines of controlled volume-weight ratio for powder metal- 35 Produ P lurgy fromian iron oxide from the group of hematite, a magnetite and mill scale which comprises: DAVID RECK Pnmary Exammer" (a) grinding the iron oxide to all. pass mesh (Tyler) W. W. STALLARD, Assistant Examiner.

with about percent passing 325 mesh and between i 

1. THE METHOD OF MAKING HIGH YIELDS OF HIGH QUALITY FINES OF CONTROLLED VOLUME-WEIGHT RATIO FOR POWDER METALLURGY FROM AN IRON OXIDE FROM THE GROUP OF HEMATITE, MAGNETITE AND MILL SCALE WHICH COMPRISES: (A) GRINDING THE IRON OXIDE TO ALL PASS 20 MESH (TYLER) WITH ABOUT 25 PERCENT PASSING 325 MESH AND BETWEEN ABOUT 25 PERCENT AND ABOUT 40 PERCENT BEING LARGER THAN 100 MESH; (B) REDUCING SAID IRON OXIDE IN A FLUIDIZED BED AT A TEMPERATURE IN THE RANGE OF 600*F. TO 1100*F., AND AT A PRESSURE IN THE ORDER OF 200 TO 650 P.S.I.G., WITH A HYDROGEN RICH REDUCING GAS BY PASSING THE REDUCING GAS UPWARDLY THROUGH THE IRON OXIDE AT A VELOCITY TO ESTABLISH FLUIDIZATION; 